Conventionally, a sandwich molded product configured from an outer layer and an inner layer contained within this outer layer, is known. As an injection molding method (co-injection method) that molds a sandwich molded product, mainly two methods, that is, a multistage molding method and a simultaneous molding method, are known. The multistage molding method is a method in which an outer layer resin is injection-filled inside a mold cavity and then an inner layer resin is injection-filled inside the outer layer resin, and the inside of the mold cavity is filled by these two resins (refer to Patent Document 1). The simultaneous molding method is a method in which outer layer resin is injection-filled inside a mold cavity and then outer layer resin and inner layer resin are injection-filled inside the previously injected outer layer resin in a laminar flow state such that the outer layer resin is on an outer peripheral side and the inner layer resin is disposed at the center of the outer layer resin, and the inside of the mold cavity is filled by these two resins (refer to Patent Document 2).